Well if you don't send the DID number to AstB then how do you expect it to know what it was? Just because you send AstB a number (e.g. ${EXTEN}@trkAstB), this does not mean that AstB will have to send the call to a *local* extension. You can modify the dial plan on AstB to forward the call to wherever you want based on the dialled number, just like you are doing on AstA.
CALLERID(num) will set the number it appears to have been called from, not the number that was dialled. You should be able to read that value on AstB if you really want to do it like that, but it seems more natural to me that you send the dialled number (the DID) as the dialled number and not as the Caller ID.
